The present invention relates to a process for making polycrystalline cubic boron nitride (CBN) from graphitic boron nitride (GBN) and more particularly to a direct conversion process for making CBN/CBN composite masses using high pressure/high temperature (HP/HT) processing conditions.
Three crystalline forms of boron nitride are known: (1) soft graphitic (hexagonal) form (HBN) similar in structure to graphite carbon, (2) a hard wurtzitic (hexagonal) form (WBN) similar to hexagonal diamond, and (3) a hard zincblende (cubic) form (CBN) similar to cubic diamond. The three boron nitride (BN) crystal structures may be visualized as formed by stacking of a series of sheets (layers) of atoms. In the low pressure graphitic structure, the stacking layers are made of plane or fused hexagons (like bathroom tile) in which the vertices of the hexagons are occupied alternately by boron and nitrogen atoms and stacked vertically such that the B and N atoms also alternate in the stacking [001] direction. In the more dense CBN and WBN crystal structures, the atoms of the stacking layers are puckered out-of-plane and the two dense structures result from variation in the stacking of the layers.
In HBN and WBN crystals the layers are stacked along the [111] direction. These layers are referred to as hexagonal stacking layers or planes. In HBN, bonding between the atoms within the layers if predominantly of the strong covalent type, but with only weak van der Waals bonding between layers. In WBN and CBN, strong, predominantly covalent tetrahydral bonds are formed between each atom and its four neighbors.
Known processes for making polycrystalline CBN compacts and mesh CBN can be generally classified in four categories as follows: (1) catalytic conversion process, a one-step process in which a catalyst metal or alloy aids in the transition of HBN to CBN simultaneously with the formation of single-crystal CBN particles or a compact thereof; (2) bonding medium process, a two-step process in which the first step comprises the conversion of HBN to CBN and the second step comprises the formation of a compact from cleaned CBN crystals mixed with a metal or alloy which aids int he bonding of the CBN into a compact; (3) direct sintering process, a two-step process which is the same as process (2) except that the compact is formed without addition of metal or alloy to aid in bonding CBN crystals; and (4) direct conversion process, a one-step process in which substantially pure HBN is transformed directly to a CBN compact or polycrystalline CBN particles without the aid of a catalyst and/or a bonding medium.
The catalytic and bonding medium processes generally are disadvantageous because the catalyst and bonding medium are lower in hardness than CBN and are retained in the resultant mass, thus reducing the hardness and abrasive resistance thereof. Particular reference can be made to U.S. Pat. Nos. 3,233,988 and 3,918,219 for a more detailed discussion of catalytically formed CBN compacts and to U.S. Pat. Nos. 3,743,489 and 3,767,371 for the details of CBN compacts utilizing a bonding medium.
A preferred direct conversion process is disclosed in U.S. Pat. No. 4,188,194 wherein a sintered polycrystalline CBN compact is made by placing preferentially oriented pyrolytic hexagonal boron nitride (PBN) in a reaction cell wherein the boron nitride is substantially free of catalytically active materials. The cell and the contents then are compressed at a pressure of between about 50 Kbars and 100 Kbars while being heated to a temperature of at least about 1800.degree. C. within the CBN stable region of the BN phase diagram. The HP/HT conditions then are maintained for a period of time sufficient for the pyrolytic boron nitride to transform into a sintered polycrystalline cubic boron nitride compact. When HBN is milled to a small particle size (large surface area), an improvement in such process is disclosed in U.S. Pat. No. 4,289,503 wherein boric oxide is removed from the surface of the HBN powder before the conversion process. Such pretreatment is carried out at a temperature in the hexagonal boron nitride thermal decomposition range and is accomplished by vacuum firing and heating under vacuum or inert atmosphere.
Variable structured multicrystalline CBN/CBN composites heretofore have involved the addition of expensive single crystal CBN to the GBN to be converted into CBN. While such variable structure can improve the performance of such abrasives in grinding applications, the expense of the single crystal CBN remains a problem. The same holds for such variable structured masses when their thermal conductivity is the property desired.